Carburetors are currently used to provide the combustion fuel requirements for a wide range of two cycle and four cycle engines including handheld engines, such as engines for chainsaws and weed trimmers, as well as a wide range of marine engine applications. Diaphragm type carburetors are particularly useful for handheld engine applications wherein the engine may be operated in substantially any orientation, including upside down. Typically diaphragm carburetors have been used with two cycle engines and there is a continuing struggle to reduce the exhaust emissions of these engines to prevent escape to the atmosphere of hazardous hydrocarbon vapors and to comply with increasingly strict governmental regulations regarding the same. Still further, engine manufacturers are continually seeking carburetors which are easier to calibrate and of lower cost.
A conventional diaphragm type carburetor has a high speed fuel circuit and an idle or low speed circuit. Typically, the high speed fuel circuit fuel jet or nozzle is disposed upstream of a throttle valve in a venturi of the carburetor body and the idle circuit has a plurality of fuel jets or nozzles some of which are disposed downstream of the throttle valve within the throttle bore. Each fuel circuit has a separate fuel adjustment needle which cooperates with a valve seat to limit the fuel flow available to the nozzles of its corresponding fuel circuit to control the amount of liquid fuel delivered from the carburetor. When the throttle valve is closed, or nearly so, such as at idle or low speed and low load engine operation, substantially no fuel is provided through the high speed fuel circuit and most, if not all of the engine fuel requirements are supplied through the idle or low speed circuit. When the throttle is wide open, fuel is fed to the engine primarily from the high speed fuel circuit of the carburetor, usually with some fuel supplied from the idle fuel circuit, although, in some systems the idle circuit may be shut off by a valve actuated by the throttle shaft.
To provide a lower cost carburetor, one solution has been to provide a single needle diaphragm carburetor which provides fuel entirely through a single fuel circuit of the carburetor. All of the fuel passes through a single needle valve orifice and out of the same series of holes or nozzles at idle or low speed and low load conditions as well as wide open throttle conditions. Holes of the series are provided both downstream and upstream of the throttle valve of the carburetor in a similar location as that of the idle fuel circuits of a carburetor having both idle and high speed fuel circuits each with a separate adjustment needle valve.
While the single needle diaphragm carburetor is of lower cost, it has poor acceleration characteristics, and an extremely rich idle calibration to partially compensate for the poor acceleration characteristics inherent to this type of system. This rich idle calibration condition can cause poor idle stability and contributes to poor stability of the engine as the engine comes down from high load conditions such as wide open throttle to a low load condition such as idle. The poor come down stability is due to the fact that at wide open throttle a greater quantity of fuel is passing through the fuel nozzles and the fuel circuit than is needed for idle engine operation such that when the throttle valve is subsequently moved from wide open throttle to its idle position an excessive amount of fuel is left in the nozzles and the fuel circuit providing a fuel and air mixture to be delivered to the engine which is excessively rich for engine come down and idle operation. This rich fuel and air mixture reduces engine stability and can even cause the engine to cease to operate.
Despite these significant shortcomings, single needle diaphragm carburetors have been successfully employed in use with piston port and relatively large displacement two-stroke engines. However, many small engine applications, such as for chainsaws, weed trimmers and the like, utilize very small displacement two-stroke engines and four-stroke engines due to the benefits in fuel economy and reduced engine emissions. The above noted shortcomings of the single needle diaphragm carburetor are particularly problematic with these mini four-stroke engines and small two-stroke engines.